1. Field of the Invention:
The present invention relates to the field of forgings.
2. Prior Art:
Forging may be defined as a process of giving metal increased utility by shaping it, refining it and improving its mechanical properties through controlled plastic deformation under impact or pressure. It is believed to be the oldest known metal working process, even being alluded to in several Old Testament accounts. "For centuries the process of hot working metal has been known and used to assure strength, toughness, reliability and highest quality in many types of products. Today the advantages of forged components assume greater importance than ever before as operating temperatures, loads and stresses increase, and reliability and toughness become more critical."
The foregoing quotation was taken from a book entitled "Forging Industry Handbook of the Forging Industry Association", and published by the Ann Arbor Press, Inc. (1966). Clearly the emphasis on forging quality has continued and will continue to take on even greater importance as the years go by in such fields as aircraft production because of the ever increasing constraints of strength, weight, absence of fatique and overall reliability in such fields.
The foregoing book provides a very good description of the prior art in general relating to various types of forging techniques,and accordingly will be referred to from time-to-time herein, both for the purposes of illuminating the prior art and for distinguishing the present invention therefrom. In terms of the definitions given in the foregoing book, the present invention is directed to impression die forging in general utilizing a closed die in the sense of distinguishing between rolling, upsetting, etc., though is not directed toward "closed die forging" as specifically defined on page 87 of the foregoing book, wherein the material being forged is to form in a cavity that allows essentially no escape of excess material, thereby effectively comprising a flashless forging process. Thus, the general type of forging for which the present invention is directed utilizes forging dies characterized by die members each having the desired die impression surrounded by a flash land, which in turn is generally surrounded by a gutter, the gutter being surrounded by a flat die surface for contacting the corresponding die surface on the mating die member. The flash land is generally a region which in cooperation with the land on the mating die defines a gap of predetermined thickness and width at the parting line through which material may be extruded during die closure to form the flash characteristic of conventional forgings prior to the trimming operation. The gutter surrounding the flash lands provides a receptacle for excess flash and/or flash remaining after previous operations and disposed or extruded outward past the flash lands as the flash lands close thereon.
In particularly simple forging shapes, forging finished dimensions could possibly be done in a single step. However, most forgings are formed utilizing at least one intermediate forging operation, generally referred to as a blocking operation, using a blocker comprising a set of dies having a cavity with a similar impression as desired on the finished forging but generally characterized by lessor detail and more generous radiuses. This reduces the required work to be performed by the finishing dies and allows the formation of an intermediate part having a material distribution, etc. chosen and varied as required to assure filling of the finishing die cavity. Generally speaking, blocking is a relatively unrestricted die forming operation in that the flow of material caused by the dies is a relatively unrestricted flow, and flash, may be considered more as a continuation of the die cavity rather than the highly restricted material flow defined in the finishing dies by the flash lands to restrict the flow of flash and provide the rapid cooling thereof, both of which cause the pressure of the material within the die cavity to greatly increase so as to encourage complete filling of the die cavity. Blocking also has a further advantage of allowing controlled working of the material to define grain orientation, etc. which will carry over into the finished forging to provide the desired directional strength characteristics. Because blockers are characterized by their relative absence of thin sections, sharp features, etc., material flow in a blocking operation is relatively modest in comparison to that experienced at least in some regions of the finishing operation, though the distribution of the material is appropriately altered to facilitate finishing. Depending upon the complexity of the forging there may be a single blocking operation followed by the finishing step, or a series of blockers may be used to progressively encourage the billet or performed stock toward a material distribution to better facilitate the finishing operation.
In forgings of various shapes and/or intricate detail it is always a problem to obtain complete filling of the finishing die cavity. In a blocker or set of blockers and a set of finishing dies which do not quite fill in one or more regions, various steps may be taken to obtain complete filling. The simplest steps generally include relieving the blocker in the appropriate area so that more material is provided on the blocked part in the region where better filling is desired, or starting with a somewhat larger stock so that the filling pressures in the finishing operation are increased, the increased material of course ultimately being ejected as additional flash during finishing. The net result is that production techniques currently in use, particularly with respect to forged parts of any complexity, will generally result in very substantial flash surrounding the finished part prior to trimming. Further, the flash typically is non-uniform, as heretofore acceptable forging techniques, even for the most demanding and precise applications, have generally not controlled or limited the amount of flash which may be obtained or assured any uniformity therein. In fact it is stated on page 101 of the foregoing book that "it should be clearly understood that any effort aimed at reducing the flash quantity or increasing the accuracy of the forging will, in most instances, lead to higher die pressures and consequently shorter die life." Thus as shown on page 264 of the foregoing book, flash formed at the sides of a forging during the forging of an automotive crank shaft of C-1046 material extends outward in a width somewhat more than half the connecting rod bearing and main bearing diameters, whereas the total flash on the connecting rod shown on page 279 of that book, relative to the size of the part, is even greater. In both cases the photos show that some flash from the blocking operation normally extends so far outward as to not contact the flash lands on the finishing dies, though in both cases, particularly the crank shaft, there is a wide region of flash shaped by and extruded through the finishing die flash lands.
Forging materials generally exhibit better ductility and strength in a direction parallel to that of the plastic elongation caused during the processing (forging) thereof. Accordingly it is common to form an elongated part such as a strut, pitman arm, etc. by the progressive elongation of a relatively short piece of stock, leading to the forging of the finished part so that the grain orientation is generally aligned with the length of the finished part.